1. Field of the Invention
This invention relates to the production of adhesives containing block polymers and more particularly to block polymer pressure sensitive adhesives (PSA) of various compositions. Many such adhesive compositions and specifically PSA compositions are well known in the art. More particularly, the invention relates to an improved method for the production of these adhesives.
2. Description of the Related Art
Since this invention is of more importance presently to PSA's, the following descriptions will refer primarily to such adhesive compositions for purposes of exemplification of the method of the invention.
Pressure sensitive adhesives (PSA) and other adhesive compositions making use of block copolymers include both linear and radial types. Generally, such block copolymers may be represented schematically as: ##STR1## wherein A blocks may comprise for example, polymerized vinyl-substituted aromatic monomers and the B blocks comprise for example, polymerized dienes having 4-12 carbon atoms or other elastomeric or rubbery polymeric component(s). In the radial polymers a polyfunctional coupling agent is included and these are well known, for example, see U.S. Pat. No. 3,639,521. In the case of PSA's, the A blocks preferably comprise polymerized styrene and the B blocks preferably comprise polymerized butadiene isoprene, or mixtures thereof or hydrogenated versions thereof. Such copolymers typically have a molecular weight in the range of about 70,000-140,000 and have about 12 to 35 wt-% styrene. Such block copolymers are available from Shell Chemical Company, Enichem, Fina and Firestone and others. A common block polymer of this type is KRATON 1102, available from Shell and is referred to herein by way of exemplification of the invention.
In addition to the block polymer constituent(s), PSA compositions will include tackifying resin(s), plasticizing oils and sometimes other optional ingredients, as is well known in the PSA art. Other block polymer adhesive compositions may include waxes, non-tacky resins and so forth.
A wide variety of PSA compositions are known and used in the art as evidenced for example by U.S. Pat. No. 5,024,667 and U.S. Pat. No. 5,037,411 all compositions of which may be produced by the method of this invention. Likewise, a wide variety of tackifiers are known as are wide varieties of the plasticizer oils. For example, typical tackifying resins include rosin, tall oil, rosin ester resins, natural and synthetic terpenes and various aromatics. Plasticizer oils are primarily hydrocarbon oils, typically paraffinic or naphthenic in character. However, other plasticizers may be used.
Most preferred for use herein, when PSA compositions are involved, are the block copolymers wherein the A block or non-elastomeric block is styrene and the elastomeric or B block is butadiene. Typically the latter is present at levels of about 50 to 80 wt % although this will vary and is not critical. The method maybe practiced with any PSA compositions and any other block polymer adhesive compositions. Block polymers of this specific type and composition are commercially available from Shell Chemical Company under the tradename KRATON 1102 (about 70 parts butadiene and 30 parts styrene) among others. Other manufacturers of block polymers for adhesives are well known. Blends of these high molecular weight copolymers with compatible low molecular weight block copolymers may also be employed. All such foregoing polymeric compositions are referred to herein as "block polymer(s)" in general and collectively.
The traditional method of preparing block polymer adhesives is to add resin and oil to a jacketed tank equipped with a standard vertical agitator. This mixture is heated to about 300.degree. F. and beads of block polymer are added. The temperature is maintained between about 300.degree. to 350.degree. F. with continued agitation. The adhesive is complete when the block polymer is completely blended into the resin/oil mixture. The time required for complete blending is usually 6 hours to 24 hours, depending on the degree of agitation. Block polymer beads disperse very slowly without a shearing and smearing action. Such long times at temperature can cause degradation of the polymer.
The commonly preferred and commercially practiced prior art method for producing block polymer PSA compositions involves the working in shear of the block polymer. This is usually initiated by adding preformed commercially available beads of the selected polymer composition to conventional thermoplastic polymer processing equipment capable of providing sufficiently high shear to blend the polymer by working it to a taffy-like melt. Small amounts of tackifier resin and oil may be added initially to facilitate the working. The block polymer is typically available commercially in a bead-like form and a variety of compositions for this purpose are available from many sources.
In the prior art, after working the polymer, (typically 30 minutes-2 hours or so), to achieve the taffy-like melt condition the main additive amounts of resin, oil and/or any other ingredients are added. The resin is preferably heated prior to being added to the melt. The resultant liquid composition is passed from the processing equipment to a holding tank or the like and then to the final container such as a drum, pillow box, or the like in which it is ultimately sold, where it is allowed to cool and solidify.
The processing equipment used for this purpose is extremely expensive in excess of hundreds of thousands of dollars, and is very high power consumptive in operation. Typically it includes such equipment as screw extruders, Mixtruders, Sigma Blademixers or the like.
More detailed information on presently practiced production methods for PSA is described in the following articles:
The article "Machinery and Equipment" of Plastics Compounding Redbook or 1987/88, is a compilation of information regarding compounding and size reduction equipment including lab and production scale blending apparatus. the articles "Liquid Resin Injection System for Continuous Mixing of HMPSA" and "Extruder Performance Over Broad Meld Index Ranges" discuss blending materials having different viscosity profiles. PA1 The article "Advance in Production and Coating Technology for Hot Melt Pressure Sensitive Adhesives-Multiruder System" discusses the multiruder system of premixer, multiruder and coater and is used for continuous production and coating of hot melt pressure sensitive adhesives for labels and tapes. PA1 The article "Continuous Compounding of Hot Melt Adhesives" discusses continuously operating blending systems capable of blending hot melt adhesives with reduced heat history. Such compounding systems involve extrusion processing. PA1 The article "Technical Aspects for Extrusion Compounding of Hot Melt Adhesives" discusses continuous processing systems for the manufacture of hot melt and pressure sensitive adhesive formulations using batch and twin screw manufacturing methods. PA1 The article "Twin Screw Extruder Process of Adhesive" discusses continuous processing hot melt adhesive compounding with block polymer formulations using multi-stage feeding of oil resins, additives and base polymer. PA1 The article "Continuous Production of Hot Melt Adhesives Aspects of Quality and Costing" discusses basic processing possibilities for producing hot melts continuously involving working from a premix and deals with four basic continuous processes. The disclosure involves multi-stage addition, pelletizing, coating, etc. Equipment for such manufacture is shown in the brochures of Welding Engineers, Inc., Eagan Manufacturing Company, Buss-Kneader, Warner and Pfleiderer, Conair and Black Clausen.
In accordance with this invention a new and improved method of preparing adhesive compositions is provided. The new method minimizes the cost of process equipment and minimizes heat degradation of the block polymer. Most importantly the process time involving the polymer is a matter of an hour or even merely minutes rather than hours. This is important with respect to reducing the heat and shear to which the polymer is subjected. Additionally, the method consumes less electrical energy and utilizes lower horsepower motors than the prior art.